3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations, also referred to as Evolved NodeBs (eNBs) to mobile stations, also referred to as User Equipments (UEs), are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a Resource Block (RB) which in its most common configuration comprises 12 subcarriers and 7 OFDM symbols, one slot. A common term is also a Physical Resource Block (PRB) to indicate the RB in a physical resource. Two PRB in the same subframe that use the same 12 subcarriers are denoted a PRB pair. This is the minimum resource unit that can be scheduled in LTE.
A unit of one subcarrier and one OFDM symbol is referred to as a Resource Element (RE) 4 as shown in the downlink physical resource 2 representation in FIG. 1. Thus, a PRB includes 84 REs. An OFDM symbol 6 including cyclic prefix is also shown in FIG. 1. The cyclic prefix makes the OFDM signal less sensitive to time dispersion of the channel. Inserting a cyclic prefix is achieved by simply copying the last part of the OFDM symbol and inserting it at the beginning of the OFDM symbol. An LTE radio subframe is composed of multiple resource blocks in frequency with the number of PRBs determining the bandwidth of the system, and two slots in time as shown by the downlink subframe 8 of FIG. 2. Additionally, in the time domain, LTE downlink transmissions may be organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes of length Tsubframe=1 MS.
Messages transmitted over a radio link to UEs may be broadly classified as control messages and data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, signaling of RBs within which the data is to be received by the UE or transmitted from the UE and so on.
In Release 8 of the 3GPP LTE specifications, with specific reference to 3GPP TS 36.211, TS 36.212, TS 36.213, the first one to four OFDM symbols, depending on the configuration, in a subframe are reserved to contain such control information, as shown, for example, by control region 10 of FIG. 2. Furthermore, in Release 11 of the 3GPP LTE specifications, an enhanced control channel was introduced, Enhanced Physical Downlink Control Channel (EPDCCH), in which PRB pairs are reserved to exclusively contain EPDCCH transmissions, although excluding from the PRB pair the one to four first symbols that may contain control information to UEs of releases earlier than Release 11 of the 3GPP LTE specifications. An illustration of this is shown in FIG. 3. In FIG. 3, the downlink subframe 12 showing ten RB pairs and configuration of three EPDCCH regions, referred to as filled with horizontal, vertical and diagonal stripes, of size one PRB pair each. The remaining PRB pairs may be used for Physical Downlink Shared Channel (PDSCH) transmissions.
Hence, the EPDCCH is frequency multiplexed with PDSCH transmissions contrary to PDCCH which is time multiplexed with PDSCH transmissions. The Resource Allocation (RA) for PDSCH transmissions exists in several RA types, depending on the Downlink Control Information (DCI) format. Some RA types may have a minimum scheduling granularity of a Resource Block Group (RBG). An RBG is a set of adjacent, in frequency, resource blocks and when scheduling the UE, the UE is allocated resources in terms of RBGs and not individual RBs.
When a UE is scheduled in the DownLink (DL) and the Downlink Control Information (DCI) message is carried by an EPDCCH, the UE shall assume that the PRB pairs carrying a DL assignment are excluded from the resource allocation, i.e., rate matching applies. Here, rate matching means that the number of output bits from the encoder is ensured to match the number of available physical channel bits. So in this context, the PRB pairs carrying a DL assignment does not have any physical channel bits available for PDSCH transmission. Rate matching is carried out by systematically removing encoded bits from the output. Which bits are removed are known at both the transmitter and receiver side. This is also known as code chain rate matching. For example, if a UE is scheduled a PDSCH in a certain RBG of size of 3 adjacent PRB pairs, and one of these PRB pairs comprises the DL assignment, the UE shall assume that the PDSCH is only transmitted in the two remaining PRB pairs in this RBG. Note also that multiplexing of PDSCH and any EPDCCH transmission within a PRB pair is not supported in Release 11 of the 3GPP LTE specifications.
The PDCCHs and EPDCCHs are transmitted over radio resources that are shared between several user UEs. Each PDCCH comprises smaller parts, known as Control Channel Elements (CCEs), to enable link adaptation by controlling the number of CCEs a PDCCH is utilizing. The number of CCEs in a PDCCH is called its CCE aggregation level, and may be 1, 2, 4, or 8 consecutive CCEs, logical sequence. The total number of available CCEs within the control region, see FIG. 2, is determined by a Physical Control Format Indicator Channel (PCFICH) configuration, the system bandwidth and the number of configured PHICH resources. Each PDCCH comprises exactly one DCI.
Multiple aggregation levels are required to support multiple DCI formats to improve resource utilization and to provide means for adapting the code rate of the DCI message to the channel quality. The DCI size varies a lot depending on the format and the channel bandwidth. PDCCHs with different aggregation levels may increase the granularity of the resource utilization, instead of a “one size fits all” solution. Higher aggregation levels may be used for broadcast control message resource allocations to provide more protection. The aggregation level for control messages may be 4 or 8, while the aggregation level for DCI messages that schedule UE specific PDSCH or PUSCH transmissions may be 1 or 2 or 4 or 8. Hence it is specified that for PDCCH, a UE has to monitor four aggregation levels of CCEs, namely, 1, 2, 4, and 8, for UE-specific search space and two aggregation levels of CCEs, namely, 4 and 8, for common search space. A search space is the collection of CCE within the total set of all CCEs in a subframe where the UE may find (i.e is searching for) its PDCCH candidates.
3GPP Technical Specifications 36.213 “Physical Layer Procedures, Release 8”, from 2008, in Section 9.1.1 describes a search space Sk(L) at aggregation level Lε{1, 2, 4, 8} which is defined by a contiguous set of CCEs given by the following:(Zk(L)+i)mod NCCE,k  (1)
where NCCE,k is the total number of CCEs in the control region of subframe k, Zk(L) defines the start of the search space, i=0, 1, . . . , M(L). L−1 and M(L) is the number of PDCCHs to monitor in the given search space. Each CCE comprises 36 QPSK modulation symbols. The value of M(L) is specified by Table 9.1.1-1 in 3GPP Technical Specifications 36.213 “Physical Layer Procedures (Release 8)”, which is replicated below.
TABLE 1M(L) vs. Aggregation Level L for PDCCHSearch space Sk(L)Number of PDCCHTypeAggregation level LSize [in CCEs]candidates M(L)UE-specific16621264828162Common41648162
With this definition, search space for different aggregation levels may overlap with each other regardless of system bandwidth. More specifically, UE-specific search space and common search space may overlap and the search spaces for different aggregation levels may overlap. See one example shown below where there are nine CCEs in total and very frequent overlap between PDCCH candidates: